The present invention relates to the field of temperature sensors and concerns more particularly a compact optical fibre temperature sensor.
Currently, optical fibre temperature sensors are divided into two categories: the first category concerns so-called extrinsic optical sensors and the second those called intrinsic optical sensors. In extrinsic optical sensors, the optical fibre is a passive element assuring a simple transmission line function and it must therefore be perfectly insensitive to the physical variable to be measured. Conversely, for intrinsic optical sensors, it is the optical fibre itself which is the element sensitive to the variable to be measured, which acts directly on the actual physical characteristics of the fibre.
There is known, in particular from French Patent No. 2 664 695, an optical fibre temperature sensor of the intrinsic type wherein the multimode type optical fibre which includes an outward connection length (from the optical source) and a return connection length (to the detection and exploitation circuit) is formed in its sensitive portion in a winding of determined curvature with several turns in the medium to be monitored. This sensors relies on the birefringence properties of a curved optical fibre. It is known that the curvature of a fibre, by inducting stress in the fibre, causes losses and thus a drop in the luminous intensity transmitted.
However, such a temperature sensor also has numerous drawbacks. First of all, the configuration of the sensitive portion of the optical fibre, wound over several turns, influences the space requirement and lifetime of the sensor. Each optical fibre in fact has a critical radius of curvature Rc for which the fibre will fracture (for example for an entirely silica fibre with an external radius r, this critical radius of curvature is equal to 100 r/3.3). Hence, the space requirement of the sensor is imposed by this minimum dimension and the lifetime of the sensor will be shorter the closer the winding diameter is to this critical fracture diameter. Further, the use of a multimode fibre generates particularly severe exploitation conditions. Indeed, a fibre of this type includes a large number of propagation modes, which greatly depend both on the optogeometrical properties of the fibre (refraction index and core radius, refraction index profile, numerical aperture), on the initial injection conditions of the optical source (solid angle and emitted wavelength source-fibre distance, axial and angular alignment), on the conditioning of the transmission line (rectilinear or curved path), and on the surrounding temperature. Hence, it is very difficult to obtain identical modal distributions from one sensor to another. Finally, in such curved optical guides, the temperature response is directly linked to the modal distribution at the input end of the fibre and it is assumed that only the first curvature lengths efficiently contribute towards generating losses, the losses virtually stabilising beyond a certain length.
The present invention thus concerns a compact optical fibre temperature sensor which nonetheless has great linearity and a large measuring range, and is arranged so as to permit industrial exploitation.
These objects are achieved by an optical fibre temperature sensor including an optical source for supplying an optical signal, a first fibre optic transmission line connected to the optical source, a sensitive portion of the optical fibre connected to the first fibre optic transmission line, a second fibre optic transmission line connected to the sensitive portion of the optical fibre, and an optical detection and processing circuit connected to the second fibre optic transmission line to receive and analyse the optical signal transmitted by the optical source and passing through the optical fibre, characterised in that said sensitive portion of the optical fibre is mounted on a plane support and bent over a determined length (N1) and with a determined curvature amplitude (A1). Preferably, the sensitive portion is periodically bent (T1).
Via this particularly simple structure wherein the sensitive portion of the optical fibre is shaped in an optical guide having perfectly determined characteristics, it is possible to obtain particularly precise temperature measurements. Moreover, the use of an optical fibre offers such a sensor complete immunity as regards perturbation of electromagnetic origin and complete security in a sensitive, and particularly an explosive, medium.
In an alternative embodiment, a second transmission line can be omitted from this optical fibre temperature sensor and replaced by a plane reflective element directly connected to the sensitive optical fibre portion, which is mounted on a plane, periodically bent (T1) support over a determined length (N1) and with a determined curvature amplitude (A1).
Depending on the optical fibre employed to form the sensitive portion, the period of curvature T1 is selected so as to satisfy either the following inequality:
2xcfx80T1xe2x89xa71/xcfx812kn1+2xcex94/xcfx81 for a step index fibre, or the following equality:
T1=xcfx81xcfx80(2/xcex94) for an optical fibre with parabolic index profile, where K=2xcfx80/xcex94 is the module of the wave vector and xcex the optical wavelength;
n1 is the refraction index of the core material of the optical fibre;
xcex94=n1xe2x88x92n2/n2 is the relative difference of the refraction indices of the core and cladding materials of the optical fibre; and
xcfx81 is the optical fibre core radius.
According to a preferred embodiment, the sensitive portion of the optical fibre of the temperature sensor according to the invention is preceded and followed by a section of fibre, respectively upstream and downstream, mounted on a plane support and periodically bent (T2, T3) over a determined length (N2, N3) and with a determined amplitude of curvature (A2, A3). Preferably, these determined lengths of the upstream and downstream sections of optical fibre are identical.
The determined periods of curvature of the upstream and downstream sections of optical fibre are determined so as to obtain optimum coupling between all the trapped modes without causing coupling with the radiated modes.
Advantageously, the optical fibre is a multimode fibre and is formed of core and cladding materials (including an outer cladding) of decreasing refraction indices. It is surrounded, at its sensitive portion, by at least one additional layer of a material having a lower refraction index than that of the outer cladding and with different optical properties as regards temperature to those of the optical fibre core. According to a first embodiment, the layer of additional material may have an optical index which decreases with temperature when the core material itself has an optical index which is constant or increases with temperature. According to a second embodiment, this layer of additional material may have an optical index which increases with temperature when the core material itself has an optical index which is constant or decreases with temperature.
Preferably, the first and second fibre optic transmission lines are interlaced to form a network of bends of low amplitude and determined period. This determined period is selected so as to obtain optimum coupling between the trapped modes, without causing coupling between the radiated modes.
Advantageously, the optical source is selected from among the following three sources: a coherent light source of the laser type, a partially coherent light source of the superluminescent diode type, or a slightly coherent light source of the luminescent diode type.
The first and second fibre optic transmission lines preferably form a single optical fibre. However, these first and second fibre optic transmission lines may also be formed of two distinct optical fibres connected by an optical guide forming the sensitive optical fibre portion.
The present invention also concerns a temperature measuring device provided with two temperature sensors such as described hereinbefore, these two sensors arranged close to each other being supplied by a common optical source and each supplying an optical output signal respectively at first and second detectors. In a first embodiment, the sensitive optical fibre portion of one of these two sensors is made insensitive to temperature in order to allow a substantially temperature constant signal to be supplied to the corresponding detector. In a second embodiment, the sensitive optical fibre portions of these two sensors have materials with temperature index variation coefficients of opposite signs, in order to allow optical signals having opposite temperature variations to be supplied to the first and second detectors, the sum of these two signals constituting a substantially temperature constant reference signal. This second embodiment allows, in particular, a reference to be obtained in order to avoid environmental perturbations.
The invention also relates to a temperature measuring device provided with a first temperature sensor according to the aforecited structure with an optical source, an outward transmission line, a single loop sensitive portion, a return transmission line and a first detector, this first sensor being intended to co-operate with a second temperature sensor whose sensitive portion is arranged in immediate proximity to the sensitive portion of the first sensor, so as to pick up all or part of the radiating luminous light emitted in proximity to said single loop and to direct it towards a second detector by means of one or more optical transmission lines, the optical signals reaching the first and second detectors having opposite temperature variations. Consequently, the pondered sum of these two signals can constitute a substantially temperature constant reference signal. Analysis of the variation signs of the two signals detected also allows effects due to temperature to be distinguished from other perturbation. Thus, any same sign variation in the two signals generated by the sensors will be attributed to effects which are unconnected to the variable to be measured. This is what happens when the transmission cable is subjected to bends causing a simultaneous drop in the output signal of the two sensors. This is also the case when the intensity of the source fluctuates. After digitising and computer processing of the signals, corrections can be made by acting on the optical source, on the signal amplification circuits or, even better, by using an appropriate algorithm. The two signals may also be subjected to arithmetical operators giving a ratiometric type result. This result can be obtained by using analogue operators or by computer processing.
In a first embodiment, the sensitive portions of the first and second sensors are bent according to radii close to one another, the two free ends of the receiving fibre being connected to a second detector via optical transmission lines. In a second embodiment, this sensitive portion of the second sensor is straight and arranged as close as possible to the sensitive portion of the first sensor, its opposite end being connected to a second detector via an optical transmission line.
In both these cases, the temperature measuring device can further include an additional optical device inserted between the sensitive portions of the two sensors and performing the function of a light concentrator, in order to increase the light transfer efficiency from the first sensor to the second sensor. This additional optical device is advantageously formed of a material whose index of refraction is close to that of the optical cladding coating the fibre core of the second sensor and slightly greater than the refraction index of the material of the optical cladding coating the core of the first sensor. In the aforecited first embodiment, this additional device has a half ring shape with a small cross-section, so as to be able to be inserted between the two bent sensitive portions of the first and second sensors. In the aforecited second embodiment, it has the shape of a circle quadrant with a rounded base of the same curvature as the sensitive portion of the first sensor, and a pointed top which comes into contact with the sensitive portion of the second sensor.
Finally, the present invention finds application with a temperature measuring device provided with a temperature sensor of the aforecited reflective type including an optical source, a fibre optic transmission line, a sensitive optical fibre portion, a plane reflective element and a first detector, this sensor further including a diffraction grating placed just in front of the sensitive portion of the optical fibre to cause part of the spectrum of the optical signal emitted by the optical source to reflect back as a reference signal a portion of the spectrum of the optical signal transmitted by the optical source, a plane diffractive element arranged just in front of this optical source allowing this reference signal to be directed towards a second detector and the optical measuring signal being directed towards the first detector after also having passed through this plane diffractive element. Preferably, the optical source is of the spatially coherent type such as a laser diode or a superluminescent diode.